1. Field of the Invention
The present general inventive concept relates to a multi-color image forming apparatus and a method of controlling the same, to correct vertical offsets when alignment errors occur between laser scanning units and development units.
2. Description of the Related Art
In general, in an electrophotographic image forming apparatus, a laser scanning unit scans a laser beam, forms an electrostatic latent image on a photosensitive body, and transfers the electrostatic latent image onto a sheet of paper, thereby printing a desired image.
In a multi-color image forming apparatus, for the purpose of development, four toners of black (K), cyan (C), magenta (M) and yellow (Y) are used. A plurality of laser scanning units corresponding to the four toners are further included.
As illustrated in FIG. 1, the multi-color image forming apparatus includes a feeding unit 10, a plurality of laser scanning units 20K, 20C, 20M and 20Y, a plurality of development units 30K, 30C, 30M and 30Y, a plurality of photosensitive drums 31K, 31C, 31M and 31Y, a transfer unit 40, a fixing unit 50, and an ejection unit 60. The laser scanning units, the development units and the photosensitive drums are individually provided corresponding to respective colors.
When a print command is input to the multi-color image forming apparatus, multi-color images are respectively formed on the photosensitive drums 31K, 31C, 31M and 31Y by laser beams emitted from the laser scanning units 20K, 20C, 20M and 20Y. When the images are formed, a sheet of paper P passes between a sheet transporting belt 41 of the transfer unit 40 and the photosensitive drums 31K, 31C, 31M and 31Y. At this time, the multi-color images respectively formed on the photosensitive drums 31K, 31C, 31M and 31Y are transferred onto the sheet by transfer rollers corresponding thereto. The sheet on which the images are transferred passes through the fixing unit 50 to apply heat and pressure such that the images are fixed. Then, the sheet is ejected by the ejection unit 60.
As illustrated in FIG. 2, the laser scanning unit 20C to scan the cyan laser beam and the development unit 30C corresponding thereto face each other. The laser scanning unit 20C includes a polygon mirror 21C, a fθ lens 22C, a reflection member 23C, an optical sensor 24C and a focusing member 25C.
The laser beam emitted from a laser diode (not illustrated), which is included in the laser scanning unit 20C, is reflected from the polygon mirror 21C. At this time, the laser diode outputs a horizontal synchronization signal. The optical path of the horizontal synchronization signal is changed by the reflection member 23C. The horizontal synchronization signal focused by the focusing member 25C is applied to the optical sensor 24C. The optical sensor 24 detects the horizontal synchronization signal and supplies the detected horizontal synchronization signal to a signal processing block (not illustrated).
After the horizontal synchronization signal is output, the laser diode outputs a laser beam to print a cyan image. The laser beam is reflected from the polygon mirror 21C and is scanned onto the surface of the photosensitive drum 31C through the fθ lens 22C along a scanning direction Sd.
The multi-color image forming apparatus includes the plurality of development units corresponding to the plurality of laser scanning units corresponding to the respective colors.
As illustrated in FIG. 3, the color laser scanning units 20K, 20C, 20M and 20Y and the photosensitive drums 31K, 31C, 31M and 31Y of the plurality of development units are respectively provided in correspondence with reference positions Pr1, Pr2, Pr3 and Pr4.
When the plurality of laser scanning units are included in correspondence with the toner colors, the scanning positions of the laser beams emitted from the laser scanning units should be aligned corresponding to the respective colors, in order to output a high-quality image.
The scanning positions of the laser beams should be respectively aligned to the reference positions in horizontal and vertical directions. For alignment in the vertical direction (sheet transporting direction), vertical offsets are respectively set corresponding to the respective colors, as illustrated in FIG. 4. The multi-color laser beams are scanned using the respective vertical offsets. For example, as illustrated in FIG. 5, when outputting four rectangular images Ki, Ci, Mi and Yi of black, cyan, magenta and yellow, which are positioned at a predetermined position of a printing medium P1 in a vertical direction, a black vertical offset Voffset[K], a cyan vertical offset Voffset[C], a magenta vertical offset Voffset[M] and a yellow vertical offset Voffset[Y] respectively having timings corresponding to distance differences of the plurality of development units are individually applied. The vertical offsets corresponding to the respective colors are previously set.
However, the laser scanning units and the development units may be positioned to be shifted in the vertical direction at a time of installation thereof. Although the laser scanning units and the development units are accurately positioned, the units may be moved in the vertical direction due to a vibration of motors which drive the polygon mirrors to deflect the laser beams and thus the relative positions between the laser scanning units and the development units may be shifted. That is, the photosensitive drums are positioned to be shifted from the reference positions Pr1, Pr2, Pr3 and Pr4.
Accordingly, the scanning positions of the laser beams which are respectively scanned onto the photosensitive drums in the vertical direction may be changed.
In order to solve such a problem, an auto color registration method of printing a specific test pattern on a printing medium was used. The positional differences of the colors in the vertical direction were checked by a result of performing the auto color registration method and the vertical offsets were corrected in order to compensate for the positional differences.
This method is troublesome because the auto color registration operation should be performed whenever a printing operation is performed for a predetermined period. In addition, since the test pattern should be printed on the sheet, the printing medium or an ink is wastefully used. Since an operation to scan the test pattern and analyze the test pattern should be performed, the load of signal processing necessary to analyze the test pattern becomes large.
As described above, when the laser scanning units and the development units disposed in the multi-color image forming apparatus are placed to be shifted from the reference positions, the scanning positions of the multi-color laser beams are changed and thus the quality of the multi-color images printed on printing mediums deteriorates.
When the auto color registration method is used in order to correct an alignment error in the vertical direction, a separate test printing operation is required regardless of an actual printing operation and thus the processing load is increased.